This b-amyloid (1-42) contains the Tottori-Japanese (D7N) mutation where Asp7 is replaced by Asn7. In vitro analysis using beta-amyloid 1 to 40-based mutant peptide reveals that the D7N mutation does not accelerate the nucleation phase but selectively promotes the elongation phase of amyloid fibril formation. The levels of protofibrils generated from D7N beta-amyloid were markedly inhibited despite enhanced fibril formation.